1. Field of the Invention
The present invention relates to a snubber circuit for a GTO thyristor and more particularly to the snubber circuit for the GTO thyristor adapted so as to eliminate the heat loss caused in a snubber resistor in the snubber circuit for the GTO thyristor due to the ripple voltages to be produced between the anode and cathode of the GTO thyristor when the GTO thyristor has been turned off.
2. Description of the Prior Art
FIG. 1 is a circuit diagram showing a GTO snubber circuit of the prior art, wherein 1 denotes a GTO thyristor (hereinafter abbreviated as GTO), 2 denotes a snubber diode, 3 denotes a snubber capacitor, 4 denotes a snubber resistor, A denotes the anode of the GTO 1, and K denotes the cathode of the GTO 1. The capacitor 3 and the diode 2 are connected in series and this series connection is connected in parallel with the GTO 1, and, the resistor 4 is connected in parallel with the diode 2. And, the forward direction, i.e., the directions from the anode A toward the cathode K, of the GTO 1 is arranged in the same direction as the forward direction of the diode 2.
Operations of the above mentioned circuit will be described in the following. When a conducting GTO 1 is turned off, the current is commutated to the snubber circuit to pass through the capacitor 3 and the diode 2, whereby the capacitor 3 is charged. At this time, the terminal voltage of the capacitor 3, substantially as it is, is applied between the anode A and cathode K of the GTO 1. The turn-off voltage build-up rate of the voltage produced at this turn-off depends on the value of the current as well as the capacitance of the capacitor 3. In order to protect the GTO 1, the snubber circuit must have a capacity to sufficiently absorb the change in the voltage waveform produced at the time the GTO 1 is turned off, and the turn-off voltage build-up rate must be kept down. Therefore, when a large current flowing through the GTO 1 is to be turned off, the capacitor 3 must be provided with a large capacitance.
When the GTO 1 is turned on, on the other hand, the discharging current from the capacitor 3 takes a value that is limited by the resistor 4, whereby an excessive current is prevented from flowing through the GTO 1. At this time the resistor 4 dissipates in the form of heat energy the energy of the electrical charges on the capacitor 3.
And, in the event of the forward voltage between the anode A and the cathode K becoming high due to occurrence of the ripple voltages or the like at the time the GTO 1 has been turned off, the resulting current will charge the snubber capacitor 3 through the diode 2, and as the anode A- cathode K forward voltage decreases, the electrical charges on the capacitor 3 will be discharged through the resistor 4.
Since the snubber circuit for a GTO thyristor of the prior art was of the above described structure, it was required to install a capacitor of a large capacitance for a large current system. Further, in the case where the ripple voltage between the anode and the cathode of the GTO was high at the turn off of the GTO, the resistor produced large heat during the discharging period of the capacitor. When, the ripple voltage and the frequency were both high, in particular, the heat loss in the resistor was very large, and therefore, the resistor had to be made in a large form, and, as a result, the external form of the apparatus including the circuit occupied a large space.